Question

1. Like most senses, the neurons that carry that carry the somatic sensations [touch, vibration, two-point discrimination, and proprioception] innervate the ________ as the last stop before reaching the cortex.

2. Like most senses, the neurons that carry that carry the somatic sensations [pain, temperature, and some remaining touch] innervate the ________ as the last stop before reaching the cortex.

3. However, neurons that carry [touch, vibration, two-point discrimination, and proprioception] information innervate the __________ before the thalamus, while neurons that carry the [pain, temperature, and some remaining touch] information pass through it, instead innervating directly to the thalamus after traversing the spinal cord.

Answer 1

A somatosensory pathway has three long neurons; primary,secondary and tertiary. The primary neuron always has its cell body in the dorsal root ganglion of the spinal nerve. The secondary has its cell body either in the spinal cord or in the brainstem and its ascending axons cross to the opposite side either in the spinal cord or in the brainstem. The axons of many of these neurons terminate in the thalamus and the others terminate in the reticular activating system or the cerebellum.

In case of touch and certain types of pain, the tertiary neuron has its cell body in the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal lobe.

In the periphery, the somatosensory system detects various stimuli by sensory receptors, such as by mechanoreceptors for tactile sensation and nociceptors for pain sensation. The sensory information like touch,pain, temperature, etc is then conveyed to the central nervous system by afferent neurons, of which there are number of different types with varying size,structure and properties.

In the spinal cord, the somatosensory system includes ascending pathways from the body to brain. One major target within the brain is the postcentral gyrus in the cerebral cortex. This is the target for neurons of the dorsal column- medial lemniscal pathway and the ventral spinothalamic pathway. It can be noted that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex. Other ascending pathways, particularly those involved with control of posture, are projected to the cerebellum, including the ventral and dorsal spinocerebellar tracts. Another important target for afferent somatosensory neurons that enter the spinal cord are those neurons involved with local segmental reflexes.

The dorsal column system and the spinothalamic tract are the 2 major pathways that bring sensory information to the brain. The sensory pathways in each of these systems are composed of three successive neurons.

The dorsal column system is primarily responsible for touch sensations and proprioception, whereas the spinothalamic tract pathway is primarily responsible for pain and temperature sensations. In the dorsal column system, the decussation takes place in the brain stem but in the later it takes place in the spinal cord level at which the information entered. The third neurons in these two pathways are essentially same. In both, the second neuron synapses in the thalamus, and the thalamic neuron projects to the somatosensory cortex.

1.Thalamus

2.Spinothalamic tract

3.  Brainstem(As medial lemniscus)